The functional anatomy of elephant trunk whiskers

Behavior and innervation suggest a high tactile sensitivity of elephant trunks. To clarify the tactile trunk periphery we studied whiskers with the following findings. Whisker density is high at the trunk tip and African savanna elephants have more trunk tip whiskers than Asian elephants. Adult elephants show striking lateralized whisker abrasion caused by lateralized trunk behavior. Elephant whiskers are thick and show little tapering. Whisker follicles are large, lack a ring sinus and their organization varies across the trunk. Follicles are innervated by ~90 axons from multiple nerves. Because elephants don’t whisk, trunk movements determine whisker contacts. Whisker-arrays on the ventral trunk-ridge contact objects balanced on the ventral trunk. Trunk whiskers differ from the mobile, thin and tapered facial whiskers that sample peri-rostrum space symmetrically in many mammals. We suggest their distinctive features—being thick, non-tapered, lateralized and arranged in specific high-density arrays—evolved along with the manipulative capacities of the trunk.

13. Can you finish on a stronger conclusion to help put your findings in the context of the general mammalian whisker sensing literature perhaps? 18. Supp video: perhaps say trunk is haptically controlled with the whiskers, rather than whiskers? You can't really see the whiskers interacting with the object in the video, and it seems that it is the trunk rather, so the text here is misleading.
Reviewer #2 (Remarks to the Author): Dieringer and colleagues have prepared and conducted a comprehensive study to describe whisker morphology, distribution, density, and innervation in two species of elephants. The study is novel in that it examines the whiskers of the trunk, rather than the skin on which previous literature has focused. The data are technically sound; however, it was difficult in some areas to assess the strength of the evidence for the conclusions since I had trouble following the sample sizes for the different study components, and the information provided on statistics in the methods was scarce. I recommend that the authors be far more explicit in the text (and potentially a table) with the sample sizes and whether their results apply to all ages, just adults, or just the calves. The study suggests that for both species, (1) whisker distribution and density are variable across the trunk with lateralization that appears to correlate with use, (2) the whisker morphology is cylindrical and not tapered, (3) whisker whisking is not a behavior supported by observations nor by whisker FSC morphology, and (4) whisker FSCs are highly innervated. Whisker density at the trunk tip seems to be higher in one species and the species show slightly different follicle morphology, although the authors could more clearly lay out how this could reflect differences in behavior or ecology. I appreciate the authors' effort to maximize useable data from collaboration with zoological facilities, which provide valuable resources for collaborative research.
I have provided detailed comments to the manuscript as a separate pdf document, and I have included an edited Word document with line numbers added to aid in my comments. I recommend that the manuscript be condensed in a few sections and improved in clarity (specifics indicated below), and the Discussion especially could be strengthened. I've also included some grammatical and syntax suggestions, but I recognize that these reflect personal writing style.
Reviewer #3 (Remarks to the Author): The paper by Deiringer et al. reports characteristics of whiskers of elephant trunks. They offer fundamental information about trunk whiskers with valid methodology, and I was surprised that they had not been studied before this study (this was well reflected in the simple, concise title of this study). Therefore, I believe this study is important to better understand not only elephant behavior but also inter-species diversity of how whisker anatomy shapes mammalian behavior. They set clear questions to be addressed in this study, which makes readers easily understand their points. The strength of this study is that they well-integrate results from both anatomical and behavioral aspects. For example, the asymmetry of the whisker trunk length is noteworthy. They give important insights that trunk whisker does not whisk, unlike rats. Their argument that elephants have dramatically improved trunk mobility over the course of evolution and therefore do not need specific mobility of whiskers is very interesting. They also give a next question about how whisker grows in elephants, which would facilitate further research to follow. Overall, their findings about elephant whiskers, which vary according to region, age, species, and behavior will give us fundamental new insights to better understand elephants. Whisker studies in mammals are highly biased in rodents, and therefore this study has an important piece to fill the fundamental gap. I only have some minor comments.
Specific comments 1. Line 269: The statement here seems different from that in the introduction, where you state that "while African elephants tend to pinch objects with their two fingers, Asian elephants tend to grasp/wrap objects with their trunk. Please clarify this point. Thank you for allowing me to review "Elephant Trunk Whiskers." I find the topic and scope to be excellent and appropriate for Communications Biology.
The data provided in this manuscript is excellent and the ecological interpretations are clear and supported (mostly*).
Throughout the manuscript review attached, I have provided suggestions on how to improve the clarity and flow of the document. Small additions like "define what a vibrissa is and what its key features (i.e., sinus?) are" may help assist future readers to navigate this interesting paper.
Overall, I found this manuscript to be very enjoyable, and left me wanting to know more about the histomorphology of the elephant trunk. This spark of curiosity is a direct result of the author's ability to show patterns between form and function.
I did have some questions about how the behavioral data and the "rat" data were included in this paper -as they were not outlined in the methods. Not included in my "comments" in the MS WORD document provided -was a question about body size and scaling. I wonder if you have found any size-related variation? Interestingly, I have found some size-related differences in vibrissa density across mammals and I have seen age-related differences in mechanoreceptor density as well. I would have liked to have seen a small comment about the size of these two elephant populations. I fully appreciate that there are negligible differences in size between African and Asian elephants, but when you include stillborn, "baby," subadult, and adult specimens in a study -SIZE is a question that immediately comes to mind.
The discussion was amazingly clear and straightforward. However, a small concluding statement about the observed variation might be informative (directions for future work, e.g., histomorphology of the non-hairy (glabrous?) ventral surface of the trunk). I think that adding a concluding statement* may be helpful for readers like myself who wanted to know more, but assumed the authors did not collect the needed data to address my questions. However, the authors did present several hypotheses throughout -that could be "re-explored" or "re-stated" in a concluding statement.
In the end, I am very impressed with the author's holistic approach to comparative functional anatomy and I very much enjoyed reading it.
4. Very focussed literature, can you also justify the wider implications of this work? Comment & Change: As already pointed out above, we revised the ms and particularly the abstract to address the broader functional implications of our analysis.

5.
Can your imaging capture the tip? Comment on accuracy/error of imaging and measuring here Comment: The referee raises an important question. It is indeed difficult to photograph or videograph the elephant trunk with sufficient resolution to resolve whiskers and whisker tips. Our efforts to visualize whiskers met with relatively good success in the elephant-cow Anchali, who is habituated to grasp fruit in a box. This is data shown in Figure 6 and our supplementary video. In this particular behavioral setting, we have a lot of footage and we are in the position to comment on whisker contacts. In other -less staged -settings, it was much more difficult to capture trunks with sufficient resolution to resolve whiskers. This is why our observations on ventral trunk whiskers (Figure 7) were only of limited photographic quality. The reasons are that whiskers are small, elephants move their trunks very fast, we limited the use of flashlights, and need to keep a distance from the animals for security reasons (even though the elephant we worked with are well habituated to humans). Change: In the revised ms we comment in more detail on the difficulties associated with whisker and trunk tip filming in the method section of the revised ms.
6. Need to clearly state sample numbers in text (I.e. animals and whiskers) for immunohistochemistry section Comment & Change: We added ns throughout the ms.
7 Need to justify only having one rat whisker, and why you have selected that particular whisker Comment: For the comparison we picked a large rat whisker, because the length was more similar to elephant whiskers. Change: We explain the logic of our comparison choice in the revised ms. (Page 10, 2 nd para).

Switching from past to present tense, needs checking throughout Comment & Change:
We rectified these inconsistencies.

DISCUSSION
9. Could lateralisation of whisker length be caused by wear associated with lateralisation of trunk-use? Please comment on that. Comment: Yes, this is, what we think is happening and actually also meant to express in our paper. In the revised ms we devote more text and explanation space to communicate this concept better. Specifically, we point out how lateralized trunk behavior ('left-or righttrunker') in elephants is expected to lead to lateralized whisker abrasion. Change: We clarified that we think that lateralization of whisker length is caused by wear associated with lateralized trunk use. (Page 9, 2 nd para).

As well as lateralization of length, how about density and innervation?
Comment: We suspect that innervation patterns are determined prenatally. Since baby elephants are clearly not lateralized, we doubt there will be lateralized innervation. Also, whisker positions appear symmetric. While trunk use is heavily lateralized much like human hand use, there appears to be no genetic bias for certain lateralization (like the human lefthemispheric speech specialization and elephants come out 50:50 as left-and righttrunkers.) Change: We did not alter the ms here, because our insights on the issue at hand are limited.
11. Nice parallels with hyrax and manatee. I think more than one Deep vibrissal nerves has also been observed in some species of cetacean -could look wider than just this group? Comment: A great suggestion. We looked at the cetacean literature and found that there is more than one nerve in bottlenose dolphins, but only one in the bowhead whale and harbor seal, according to Change: We added this information and included the paper of Gerussi et al. We further added information about another species with more than one deep vibrissal nerve, the rhesus monkey.
12. Difference in thickness of manatee and elephant whiskers is likely due to manatees being aquatic (see Dougill's paper) Comment: We agree and also note that the referee points out an excellent reference that we had missed.  16. Figure 3a -needs a better image of rat whisker, you cannot really see it Comment: We agree. We wanted to keep a picture where the three whiskers are photographed together to highlight the difference in shape and thickness. Change: We enlarged the picture and exchanged it for the unedited version, to make the rat whisker as visible as possible next to the elephant whiskers.

Some beautiful figures here
Comment: Thanks. Change: None.

SUPP VIDEO
18. Supp video: perhaps say trunk is haptically controlled with the whiskers, rather than whiskers? You can't really see the whiskers interacting with the object in the video, and it seems that it is the trunk rather, so the text here is misleading. Comment: We think the referee's criticism of our task description was justified. Change: We added information to the description of the behavioral task to clarify that we do not know to what extent whiskers are involved in the task at hand.

Reviewer #2 (Remarks to the Author):
Dieringer and colleagues have prepared and conducted a comprehensive study to describe whisker morphology, distribution, density, and innervation in two species of elephants. The study is novel in that it examines the whiskers of the trunk, rather than the skin on which previous literature has focused. The data are technically sound; however, it was difficult in some areas to assess the strength of the evidence for the conclusions since I had trouble following the sample sizes for the different study components, and the information provided on statistics in the methods was scarce. I recommend that the authors be far more explicit in the text (and potentially a table) with the sample sizes and whether their results apply to all ages, just adults, or just the calves. The study suggests that for both species, (1) whisker distribution and density are variable across the trunk with lateralization that appears to correlate with use, (2) the whisker morphology is cylindrical and not tapered, (3) whisker whisking is not a behavior supported by observations nor by whisker FSC morphology, and (4) whisker FSCs are highly innervated. Whisker density at the trunk tip seems to be higher in one species and the species show slightly different follicle morphology, although the authors could more clearly lay out how this could reflect differences in behavior or ecology. I appreciate the authors' effort to maximize useable data from collaboration with zoological facilities, which provide valuable resources for collaborative research. Comment: The referee gives an overall positive assessment of our study and provides a concise summary of our findings. We are thankful for this positive assessment. The referee also criticizes that the sample size underlying our observations is not always clear. Change: We added info on sample sizes and statistics throughout the ms.
I have provided detailed comments to the manuscript as a separate pdf document, and I have included an edited Word document with line numbers added to aid in my comments. I recommend that the manuscript be condensed in a few sections and improved in clarity (specifics indicated below), and the Discussion especially could be strengthened. I've also included some grammatical and syntax suggestions, but I recognize that these reflect personal writing style. Comment: It is obvious to us that the referee put a huge amount of work into our ms and we found almost all criticisms justified. Change: We implemented the referee suggestions point-by-point as indicated below.

Principal strengths of the manuscript:
• The authors have combined multiple data streams (behavioral, immunohistochemical, morphological) to comprehensively describe elephant whiskers. I truly appreciate the time and effort it takes to combine these techniques that attempts to link structure and function. Comment: The referee appreciates our combined methodology. Change: None.
• The authors must be commended for creating such beautiful figures! They were a pleasure to look at and informative. • I appreciate that the authors provide data in a public repository -a great practice! Comment: The referee comments positively on our Figures and the repository. Change: None.

Principal uncorrectable weaknesses of the manuscript:
The authors obtained a range of ages and sexes for this study, but it seemed to me that sampling for some data streams may be bit biased -for example, FSC length seemed to be only sampled in calves. This is often the nature of this type of research with something other than a lab-reared model organism, and while it doesn't reflect poorly on the quality of the existing data, I have suggested places where the authors could be more explicit about the limitations for their conclusions. I did have difficulty tracking sample sizes for each arm of the study (which is something I've listed as a suggestion to improve below), so it's possible the sampling isn't as biased as it seems. Comment: The referee notes that there are uncorrectable weaknesses in the underlying samples and that our material is not as controlled and complete as histological material obtained in an elective fashion from lab-reared animals. We agree the referee has a point concerning the limitations of our material. Having said that the referee's point is well taken, we would like to add that the material entering our study (histological material from ≥ 10 elephants) is substantial in comparison to other work in the elephant field. For example, the most significant study on the somatosensory histology of elephant trunk tip (Rassmussen & Munger 1996) is an n = 1 study (with respect to histological samples). Change: We added info on sample sizes throughout the ms. We pointed out the limitations of our material in the discussion (page 14 2 nd para).

Principal correctable weaknesses of the manuscript:
• I recommend the authors develop a stronger title that provides information about either the methods used or the results/conclusions Comment: We debated various alternative titles and initially had also favored more complicated titles. In the end, however, the authors favored the current title and we'd prefer not to change it. Change: None.
• I suggest the authors provide some broader context statements about the study system and their findings in Introduction and Abstract. Right now that context doesn't come through as clearly as it could. Why are elephants interesting to examine and why might the whiskers be interesting? I think the authors could emphasize more the uniqueness of the trunk as a way to explore if patterns we see in other tactile whisker specialists is maintained in this specialized structure, as well as the importance of contributing data from non-model, non-lab-reared organisms to the literature. Comment: The referee criticizes our abstract, a comment that we take seriously, specifically because it aligns with similar criticisms of referee 1. Change: We entirely rewrote the abstract. We removed data from our summary and made an attempt to provide more context.
More details should be provided in the Methods, especially for (1) Statistics section, which only provides information on the language and packages used rather than the tests, and (2)

Any sections that could be reduced or condensed?
• I suggest being consistent with use of whiskers or vibrissae throughout manuscript -either is fine but the use of both may be confusing. Comment: We agree. Change: We now use whisker throughout the manuscript consistently wherever it doesn't interfere with the existing terminology from the literature.
• I recommend the authors adjust verb tense and change tense from passive to active whenever possible to make sentences more concise. I've provided examples below, but revising instances of these throughout the manuscript will improve conciseness. ○ "Elephants are almost constantly engaging their trunk" can be simplified to "Elephants constantly engage their trunks" ○ "Scanning of single FSCs was done by embedding them in a …" can be simplified to "We scanned single FSCs by embedding them…" Comment & change: The referee is right, we implemented these suggestions.
• I recommend the authors adjust verb tense and change tense from passive to active whenever possible to make sentences more concise. Examples provided below, but I would strongly recommend editing these throughout the manuscript. Comment: We agree. Change: We removed passive expressions, wherever adequate.

Abstract:
Line 54 No hyphen needed between whisker and density when used as a noun Comment & Change: Done.
Line 54-55 Please provide formal scientific names at first mention of the common names of the two elephant species tested Comment: We mention the scientific names of the three extant species early in the text, but not at the very beginning of the abstract. We mention them in the introduction, when we discuss species differences in trunk morphology. Change: None. We think the current order makes more sense than an earlier mentioning of the scientific names.

149-151
Were counts performed by the same human or different humans? If the latter, how did you assess inter-observer reliability? Comment & change: The counts were performed from one the authors. We agree that the inter-observer reliability is an important factor to consider for the whisker count. We let a coauthor do a recount on two of the samples with a deviation of 5% and 11% in both directions. Line 149 Here is a place you could specify sample size -"For every specimen (n = XX)" Comment & Change: We added this info.
Line 152 I recommend that "baby elephant" should be referred to as "calf" throughout the manuscript, but I recognize that published work exists using this phrase, so happy to leave this up to the authors' discretion. Comment: We agree that the term baby sounds colloquial. Change: We replaced the term baby with newborn throughout the ms.

155-159
Sampling needs to be clarified for whisker thicknesses" You state the "for each specimen" then say that this was only performed on two adults from each species. To clarify, you can state " For two adults from each species, the thicknesses of ten whiskers were measured…" Comment & Change: We agree and added this info.

162-171
Please provide details on the number of behavioral trials you completed for each individual, and thus, videos that you analyzed to assess behavior. Comment: The whiskers did not move in all high-quality videos of fruit retrieval (n > 40 from experiments with one individual). We verified this visual impression in n = 5 trials with quantitative tracking. This analysis included whiskers from both sides of the trunk. Change: We included the number of videoclips and the number of whiskers we analyzed in the corresponding method section. Line

238-240
The authors must provide more information on the statistical tests they used, any relevant assumption checks to assess significant differences between species, and how the data + error are reported (ex. means + sem).

Comment & Change:
The referee is right, as already pointed out above, we added our statistical testing procedure in the corresponding method section Line 251 Gentle reminder that data should be plural. You use it correctly in the line before, so just carry that over into this sentence.

Comment & Change:
The referee is right, we corrected the sentence.

Results
Great use of headers throughout the results! I would recommend adjusting the following to include the result in the statement, similar to your other headers in this section: "Elephant trunk whisker thickness and geometry", "Anatomy and morphological variety of trunk whisker follicles", "Ventral trunk ridge whisker arrays and their behavioral contact patterns" Comment & Change: A great suggestion that makes the results read more cohesive.
Line 260 What do you mean by more prominent? Longer, thicker, etc? Please describe this with regard to the attributes of the whiskers. Based on the figure from the side view it does seem that they are longer, although they also look darker (which would make them appear more prominent on the white background). Comment: The whisker of African elephants are more obvious to the observer, a property, to which we refer as 'prominent'. Change: None.

269-270
This is the first mention of the "fingertip" which I take to mean the peripheralmost part of the tip? I would recommend defining this more explicitly when you first mention it as an anatomical feature. Also, you mention that they pinch objects with the fingertip, but this seems like it will become more of a Discussion point than a Results point unless you are referring to specific observations of elephants included in this study. Comment: We agree. Change: We define fingertip in the revised ms. Line

270-272
The authors show whisker density in Fig 1e/f but then compare absolute number of whiskers in Fig 1g. The absolute numbers are difficult to interpret, as it's not clear how this accounts for any differences in surface area between the species. The authors should report density, in addition to absolute counts, and run the analysis on density to interpret the observed differences. Comment: We understand the referee's reasoning behind this comment. We provided whisker density information for n = 2 trunk tips already in our original submission (Figure 1e,f). As can be observed in these panels whisker density varies strongly across trunk tip. We therefore think that overall averages of whisker density will provide the reader only with little additional information. Change: None.

Line 275 Similar to my comment in Line 260 -it's not clear to me what "prominent" means. I recommend the authors explicitly state what they mean by this and ensure that it coincides with statistical analyses.
Comment: Please see our response above. Line 279 "Accordingly" seems to be an unnecessary transition here.
Comment & Change: We agree and rewrote this sentence Line

280-282
The word "such" seems unnecessary and a bit jarring in these sentences.

Comment & Change:
We agree and replaced such by this.
Line 291 I recommend the authors be more explicit here. Rather than "showed whisker length lateralization that is expected from such behavior…" please state what that lateralization actually was.

Comment & Change:
We agree and explain the connection by behavioral lateralization and abrasion in the revised text. Line

295-298
The way this is phrased makes it sound like you are comparing your results to those in rats from other studies based on this phrase: "an observation that is common in rats". If you are comparing your results to the rats you examined in this study, then please make it clear. If you are comparing your results to the rats examined in other studies, then this comment would fit better in the Discussion. Comment & Change: We agree and added the expression 'in other studies on rats'.

299-300
This sounds too speculative, which is odd to include in Results, unless you actually measured whisker growth rates in elephants. Comment & Change: We removed the speculative sentence.

Similar to my comment about "prominent" -thick and sturdy seems open to interpretation. I would recommend concluding that the whiskers have a cylindrical shape and are thicker than rats in both species, and variable thickness across the trunk tip in African elephants.
Comment: We understand the reasoning behind the referee's criticism. Still, we think that descriptive adjectives such prominent, thick and sturdy allow the reader to form more clear impression of elephant whiskers. Change: None.

322-340
Are the microCT scans in this section different than the ones mentioned in the previous section (Elephant trunk whisker thickness and geometry)?
The authors mention the rat in the previous section but focus that section on elephants. I actually wonder if including the rat microCT is necessary at all -the only result the authors discuss from it is that the elephant follicles are much larger. This doesn't seem to be a super informative result since it may be confounded by body and appendage size. If the authors have an H&E-stained whisker follicle, that may be helpful to include, especially since the authors make a note that the elephants lack vibrissal capsular muscles that rats have. If not, I would recommend either (1) discussing the similarities/differences between the microCT scans of the rat and elephant beyond absolute size, or (2) removing the rat microCT from the results.
Comment: We think that the side-by-side comparison of elephant and rat follicle is instructive for the reader, in particular for the large community of scientists familiar with rodent whiskers. Change: None.
Line 328 When mentioning other species in this line, the authors should provide references. In contrast to my point below, I can see how this is a Results point because the authors are using a reference to other species to indicate "typical" FSC structure. Comment: We agree. Change: We added references that include the typical FSC structure of other species (mouse, rat, hyrax).

333-345
"seen in other species, such as rodents" seems to be a Discussion point unless the authors have an H&E-stained section of a rodent whisker from this study they can add to the figure to point out the differences. Comment: We agree. Change: We removed the reference to other species. Line

339-340
The authors provide information about the statistical test in the figure caption, but this information should also be provided in text when stating significance.

Comment & Change:
We agree and added the statistical information also in the text.

341-342
These summarized results don't seem to match the text. Being large is a relative term and likely related to body size, appendage size, and whisker geometry/length. To me, it seems the results of this section are (1)  Line

358-460
This should be moved to the methods with clarification on whether axon numbers were counted in one individual (and age) or multiple individuals. Comment: We agree. Change: We added this info and moved the sentence. Line

376-402
All parts of this section that are not results should be moved to the Methods section "Videography and whisker tracking", along with more clarification on how many whiskers were tracked for the individual. The methods should not be repeated here. Line 396-397 includes what seems like a Discussion point.
Comment: We disagree and think that the findings described here are difficult to understand without the direct methodological context. Change: None. Line

405-407
What was the surface area of the trunks? That seems important to scale the absolute number of whiskers.

Comment & change:
We do not have this information for many of our specimens and this data is also not easily derived from photographs. A rough idea of trunk surface area can be deduced from Figure 7a-d, where we show photographs of both the dorsal and ventral side of newborn elephant trunks.
Line 414 I'm not sure I agree with the statement that whiskers are distributed more evenly on the dorsal trunk side. Although there isn't a distinct two-row pattern as in the ventral trunk, there still seems to be a pattern, as the authors, note from the peripheral to the proximal trunk (although the pattern is different between the species). Comment: This comment is a difficult call for us. We do not disagree with the referee that there might be some patterning in dorsal trunk whiskers. Still, the pattern is not entirely clear and our expression 'more evenly' was a deliberate attempt to stay neutral and not to overinterpret our observations. Change: None. Line

419-429
Much of these lines should be moved to the Discussion. You should mention that you personally observed and photographed elephant grasping behaviors (I'm assuming the photos in e-f are the authors'), but make sure to include that in the Methods, as well.

Comment & Change:
We agree that we report interpretations and methodological detail in this section. Still, this information is helpful in understanding the results presented here and this is why we would like to keep it in place.

428-429
"Impressive whisker density" is hard to interpret. The ventral ridge appears to have a much higher density than the rest of the trunk, which coincides with observations from the microCT scan. Whether or not this is impressive is subjective. Comment: We agree. Change: We rewrote this sentence. Line

434-435
The main conclusion seems to be that whisker density varies across the trunk region in elephant calves, which seems to align with observed adult elephant grasping behaviors. We agree that arrows in e and f would add clarity. As a and b should provide an overview of a typical section rather than emphasizing a specific area, we didn't add any arrows there. Change: We added arrows in e and f for emphasizing important structures.
D: This caption needs more information about the number of elephants these data represent. Comment: As only a few of our samples had sufficient tissue quality to be used for counting the axons from immunohistological stainings the axon count was restricted to one baby Asian elephant.
Change: We added this information in the figure caption as well as the methods and the corresponding results paragraph. Fig. 6 I would recommend adding "vibrissae" to "elephants do not whisk" as in "elephant vibrissae do not whisk" to the first lien and providing an arrow for the tracked whisker in c and g to match the arrows in d and h.

Comment & Change:
We agree and changed the heading accordingly. Additionally, we added the arrows in c and g.

Reviewer #3 (Remarks to the Author):
The The referee summarizes our results and notes that it is surprising that we do not already know about a range of findings that we document. This assessment resonates with the motivation of our study, name that the tactile biology of elephants has been neglected. The referee appears to appreciate our work and we are thankful for this positive assessment. Change: None.

Specific comments
1. Line 269: The statement here seems different from that in the introduction, where you state that "while African elephants tend to pinch objects with their two fingers, Asian elephants tend to grasp/wrap objects with their trunk. Please clarify this point. Comment: We agree that there is an apparent contradiction in the terms that were used. Change: We added an explanatory qualifier in this sentence.
2. Line 340: Figure 4i. Is this data from African or Asian elephants? Please clarify this in the figure legend. This comment is applied to Figure 5d.

Comment & Change:
The referee is right, this information was missing and we added it.

Reviewer #4 (Remarks to the Author):
Please see the attached annotated MS word document of the manuscript.

Comment & Change:
We implemented the suggested changes and highlighted the changes in the revised ms.
Thank you for allowing me to review "Elephant Trunk Whiskers." I find the topic and scope to be excellent and appropriate for Communications Biology. The data provided in this manuscript is excellent and the ecological interpretations are clear and supported (mostly*). Comment: The referee finds our topic and scope fitting and is overall very positive about the ms. We are encouraged by these comments. Change: The referee adds the qualifier mostly, which we address below when dealing with the more detailed comments.
Throughout the manuscript review attached, I have provided suggestions on how to improve the clarity and flow of the document. Small additions like "define what a vibrissa is and what its key features (i.e., sinus?) are" may help assist future readers to navigate this interesting paper. Comment: These detailed comments offer a trove of insights and we think that our ms greatly improved by the revisions that followed from these comments. We are thankful for the hard work that the referee put in here. Change: All these comments were addressed point-by-point.
Overall, I found this manuscript to be very enjoyable, and left me wanting to know more about the histomorphology of the elephant trunk. This spark of curiosity is a direct result of the author's ability to show patterns between form and function. Comment: We put in an effort to show as much of the trunk as possible; we are glad that the referee enjoyed our style of reporting. Change: None.
I did have some questions about how the behavioral data and the "rat" data were included in this paper -as they were not outlined in the methods. Not included in my "comments" in the MS WORD document provided -was a question about body size and scaling. I wonder if you have found any size-related variation? Interestingly, I have found some size-related differences in vibrissa density across mammals and I have seen age-related differences in mechanoreceptor density as well. I would have liked to have seen a small comment about the size of these two elephant populations. I fully appreciate that there are negligible differences in size between African and Asian elephants, but when you include stillborn, "baby," subadult, and adult specimens in a study -SIZE is a question that immediately comes to mind.

Comment & Change:
In this broad comment the referee notes potential problems with our sample. We tried to address this concern throughout the ms by providing more detailed ns and statistical detail (as also requested by the other referees).
The discussion was amazingly clear and straightforward. However, a small concluding statement about the observed variation might be informative (directions for future work, e.g., histomorphology of the non-hairy (glabrous?) ventral surface of the trunk).
I think that adding a concluding statement* may be helpful for readers like myself who wanted to know more, but assumed the authors did not collect the needed data to address my questions. However, the authors did present several hypotheses throughout -that could be "re-explored" or "re-stated" in a concluding statement. Comment: The conclusion needs to be rewritten as we already agreed. Adding open questions to the conclusion is an interesting idea. Change: We rewrote the conclusions para.
In the end, I am very impressed with the author's holistic approach to comparative functional anatomy and I very much enjoyed reading it. Comment: Another very appreciative comment, which we read with pleasure. Change: None.